This invention relates generally to conditioning of oxide or scale on a metal surface; more particularly on a strip of metal, and yet more particularly, to conditioning of oxide surfaces or scale on a stainless steel strip. Stainless steels are ferrous alloys containing more that about 10% chromium for the purpose of enhancing corrosion and oxidation resistance. Some stainless steels also contain nickel, molybdenum, silicon, manganese, aluminum, carbide formers and other elements. This invention is also applicable to families of alloys including superalloys where nickel is the predominant element, titanium alloys and cobalt alloys. In even more particular aspects, this invention relates to aqueous spray conditioning.
Descaling of metal strip, especially stainless steel strip, has taken many forms in the past. The simplest technique involves only the pickling of the strip in mineral acid such as sulfuric acid, hydrochloric acid, hydrofluoric acid, nitric acid, or mixtures thereof. This may work with some grades of stainless steel with very light scale; however, in most cases more is needed than just an acid pickle. In those cases, various compositions and techniques have been developed to condition the scale before acid pickling. Typical compositions for scale conditioning include mixtures of alkali metal hydroxides and alkali metal nitrates with various other additives, such as alkali halides, carbonates, and/or other oxidizing agents. These are often referred to as descaling or scale conditioning salts. A conventional technique for using such compositions is in the fused anhydrous state in a pot at elevated temperatures, e.g. 800xc2x0 F. to 1000xc2x0 F., through which the strip is passed, followed by an acid pickle. While this works well in many cases, nevertheless there are certain drawbacks to this technique in some instances. For example, the bath has to be maintained at elevated temperatures, which may be energy intensive. Also, the fused caustic baths require submerged rolls which may be difficult to maintain, and can cause marring of the surface of the strip being descaled. Additionally, there is the issue of drag-out of the fused composition, i.e. as the strip exits from the pot of fused composition, it carries a certain amount of the fused composition with it, especially at high strip speeds. Furthermore, fused bath compositions are limited to compounds that have long term stability at elevated temperatures.
Other techniques for descaling are disclosed in commonly assigned U.S. Pat. No. 3,126,301, issued Mar. 24, 1964, entitled xe2x80x9cMolten Salt Spray Process for Descaling Stainless Steelxe2x80x9d and U.S. Pat. No. 5,272,798, issued Dec. 28, 1993, entitled xe2x80x9cMethod and Apparatus for Descaling Metal Stripxe2x80x9d. These patents disclose methods and structures for spraying fused caustic-containing compositions onto a moving strip of steel to condition the scale, after which the scale is pickled off These offer several advantages, in some instances, over the technique of using a pot of fused material. However, they too have some drawbacks in some instances. They require high temperature nozzles, and the composition must be maintained at elevated temperatures, e.g. 800xc2x0 F. to 1000xc2x0 F.
Thus, there is a need for a relatively inexpensive, low temperature, and efficient technique for conditioning scale on metal surfaces, especially on stainless steel strip, or the like.
According to the present invention, a composition and apparatus and method of using the composition for aqueous spray descaling or conditioning of scale or oxide on metal surfaces is provided, especially stainless steel strip or the like, in one embodiment, although it can be used to descale or condition oxide or scale on other work pieces such as metal bar, or even discrete objects. An aqueous solution having a base composition of an alkali metal hydroxide, such as sodium hydroxide, potassium hydroxide, or a mixture of alkali metal hydroxides such as sodium hydroxide and potassium hydroxide is used. The aqueous solution may contain certain additives to improve the descaling performance of the salt. In one embodiment, the solution is used to condition the scale or surface oxide on a strip of stainless steel. The strip of steel is at a temperature between the melting point of the alkali metal hydroxide in anhydrous form and a temperature at which the Leidenfrost effect appears. One or more nozzles is provided to spray the solution, and the heated strip is passed by the nozzle or nozzles where the solution is sprayed on the surface or surfaces of the strip that have the scale or oxide. The invention also includes the apparatus and control thereof for the spraying of the solution.